In accordance with Telecommunications Industry Association (TIA) standard IS-95, the CDMA (Code Division Multiple Access) cellular radio system comprises a set of mobile stations and base stations. In the CDMA system, each base station continuously operates a pilot channel. If the base station has several sectors or beams, each sector or beam of the base station typically operates a specific pilot channel. It is previously known to use a pilot channel for identifying a base station, or a beam or sector of the base station. Each pilot channel signal typically comprises a spread-coded signal without data modulation that is continuously sent by the respective base station signal to its coverage area. The mobile station may identify the base station transceiver units by the pilot channel signal because the phases of the spreading codes in the pilot channel signal differ from one another.
In operation, upon power-up or entry into the system, each mobile station must typically initially acquire a pilot channel, typically the strongest pilot channel, of a local base station. Upon acquiring the pilot channel, when the mobile station does not have a connection on a normal traffic channel, the mobile station operates in an idle state by monitoring the paging channel of the local base station operating the respective pilot channel. Then, when necessary, the mobile station switches operation to an access state by sending messages on an access channel to the local base station, and continues to monitor the respective paging channel. When a connection is established between the mobile station and the local base station, either the mobile station or the local base station sends a connection establishment request. In the CDMA system, a specific Random Access Channel (RACH) is used as an access channel on which the mobile station initiates call establishment, such as by sending a connection establishment request. The local base station responds to the messages of the access channel on Forward Link Paging Channel.
When a traffic channel connection is to be established or when responding to a connection establishment request of the base station, the mobile station sends, via the access channel, a connection establishment message to the base station that forwards it to other parts of the system, typically to a base station controller where traffic channel resources are allocated to the connection. Subsequently, then, the mobile station and base station can communicate over the traffic channel. However, as the mobile station moves through the system, it typically continuously searches, during ongoing communications, for stronger pilot channels of base stations located near the base station, such as to determine whether to hand-off communications from one base station to another.
Also during ongoing communications, the mobile station maintains an active set, a candidate set and a neighbor set of pilot channels for a number of base stations. In this regard, the active set is a set of pilot channels operated by respective base stations that have assigned respective traffic channels to the mobile station. The candidate set is a set of pilot channels that have been received with sufficient strength by the mobile station to be successfully demodulated, but have not been placed in the active set by the base station. The neighbor set, on the other hand, is a set of pilot channels operated by respective base stations that are probable candidates for handoff. Normally, the neighbor set contains pilot channels of base stations that cover geographical areas near the mobile station. Once a pilot channel has been selected into the active set, the traffic channel (including the forward and reverse traffic channels) are capable of conveying digital voice and/or data to and/or from the mobile station, respectively.
In wireless channels, where signals can arrive at different times following different paths, the received signals can experience large amplitude and phase variations due to the interference of the different paths. In this regard, terrestrial wireless signals transmitted from or to a mobile station may be reflected from the terrain, fixed or mobile objects in the propagation path such as buildings or vehicles, or from a discontinuity in the atmosphere. If the energy of the reflected wireless signal is not significantly absorbed and/or attenuated, a plurality of different propagation paths for the wireless signals between the transmitter and receiver, referred to as multipath signals, are created. These different propagation paths, in turn, allow the wireless signals to “bend” around corners and propagate beyond terrain features and objects obstructing the line-of-sight between the base and mobile stations.
In a multipath environment during ongoing communications, non-orthogonal or noise contribution can cause degradation in multipath pilot and traffic channel signals and the resulting output of a rake detector in the CDMA receiver of the mobile station. The multipath pilot channel signal has the greatest likelihood of interfering with the other channels because it represents approximately twenty percent of total transmitted base station power. To reduce interference in such a system, techniques have been developed for suppressing interference caused by the pilot channel signal. More particularly, techniques have been developed for suppressing interference caused by one or more of the active set of pilot channel signals during ongoing communications between the mobile station and one of the active base stations over a respective traffic channel. In this regard, many such techniques suppress interference in the traffic channel of the local base station caused by the multipath local pilot channel signal, and/or interference in the each path of the local pilot channel signal caused by each other path of the local pilot channel signal.
As will be appreciated, when mobile station operates in the idle and access states, the active set of pilot channel signals includes only the pilot channel signal (referred to as the local pilot channel signal) of the local base station. As such, conventional techniques do not attempt to suppress interference from the pilot channels of other base stations since they are not included in the active set. When the mobile station operates in the idle and access states, however, other pilot channels can still interfere with the local pilot channel signal, such as those pilot channels with a signal strength sufficient to be included within the candidate set. Current techniques for suppressing interference, however, are not adapted to suppress interference caused by such pilot channel signals.